Implantable cardiac pacemakers are a class of cardiac rhythm management devices that provide electrical stimulation in the form of pacing pulses to selected chambers of the heart. (As the term is used herein, a pacemaker is any cardiac rhythm management device with a pacing functionality regardless of any additional functions it may perform such as cardioversion/defibrillation.) Most pacemakers are used in the treatment of bradycardia by enforcing a minimum heart rate. If functioning properly, the pacemaker makes up for a heart's inability to pace itself at an appropriate rhythm. Also included within the concept of cardiac rhythm is the manner and degree to which the heart chambers contract during a cardiac cycle to result in the efficient pumping of blood. Pacemakers have been developed which provide electrical pacing stimulation to one or both of the atria and/or ventricles at single or multiple sites during a cardiac cycle in an attempt to improve the coordination of atrial and/or ventricular contractions, termed cardiac resynchronization therapy. Such multiple paces are usually delivered to a heart chamber during a cardiac cycle with a pacing mode similar to that used for bradycardia pacing.
In order for a pacemaker to be effective, the paces delivered by the device must achieve “capture,” which refers to causing sufficient depolarization of the myocardium that a propagating wave of excitation and contraction result (i.e., a heart beat). A pacing pulse that does not capture the heart is thus an ineffective pulse. This not only wastes energy from the limited energy resources (battery) of pacemaker, but can have deleterious physiological effects as well, since a demand pacemaker that is not achieving capture is not performing its function in enforcing a minimum heart rate and/or providing resynchronization therapy. A number of factors can determine whether a given pacing pulse will achieve capture, but the principal factor of concern here is the energy of the pulse, which is a function of the pulse's amplitude and duration. The minimum pacing pulse energy necessary to achieve capture by a particular pacing channel is referred to as the capture threshold. Programmable pacemakers enable the amplitude and pulse width of pacing pulses to be adjusted, along with other parameters. It is common practice to determine the capture threshold by initially pacing with a high energy to ensure capture and then progressively lowering the pacing pulse energy during a sequence of cardiac cycles until capture is no longer achieved. The pacing pulse energy can then be adjusted to an appropriate value in accordance with the determined capture threshold by setting it equal to the capture threshold plus a specified safety margin. Ideally, the safety margin should be large enough to reliably pace the heart but small enough to minimize energy drain and thus prolong battery life.